a question about current

[brendandwyer]

forgive my newbie-ness:

I am attempting to get a grasp on the relationships in Ohm's Law

If i want to take a +-15vdc 230mA supply, and power a device that requires +-15vdc but onlt 5.3mA, is this achieved by simply putting a resistor of 353 Ohms in series?

This seems counter-intuitive to me, however when i calculate it, that's what i get.

The voltage stays the same, but the current can be lowered?

thanks you guys

 

[clintrubber]

The 230mA is what your supply CAN deliver, but won't if the load is less (=less demanding).
Both the supply and load 'are' 15V, so connect them and if you were to put a current meter in series with the load you'd obsere that the load determines the actual current that is drawn from the supply.
Note that the supply now draws less from the mains as well, since it hasn't work as hard as when at its max capability.

No need for additional resistors.

BTW, if you wanted to tax the supply to its max you'd be putting the additional resistor in parallel to the actual load. But to spend additional power like that is a bad idea: waste of power and probably less well regulated juice for the load since the supply has to work harder now.

 

[brendandwyer]

wow. that's F*ing cool!

again, i'm new. but that blows my mind

Thanks so much!

 

[clintrubber]

[quote author="brendandwyer"]wow. that's F*ing cool!

again, i'm new. [/quote]
Welcome & enjoy ! (and please add location in your prodfile, will come in handy one day :wink: )

but that blows my mind

All clear now ? Just tell.

 

[PRR]

The wall outlets in your house are "120V 20A" or 2,400 Watts.

You have a 120V 60 Watt lamp. What do you do? You plug it in. Although a lot of current is available, the lamp only sucks the 0.5 Amps it was designed to pull.

The "120V 20A" wall outlet is really a 120V 0.2 ohm source. If you pull 0.5 Amps it drops 0.1V to 119.9V; if you put a 1,200 Watt (120V 10A) toaster on the wall drops 2V to 118V. You "can" connect ten toasters, try to pull 100 Amperes. The wall voltage will tend to drop 20V, to 100V, and you won't get your expected 100 amps. More to the point, the wires in the wall are rated 30 Amps, the outlet is rated 20 Amps, so there is a fuse (or circuit breaker) to disconnect heavy loads. If not, then something smokes.

Your supply has a "230mA" rating. What happens if we pull more? Will it sag, smoke, or cut-out? We don't know. But since we only expect a 5.3mA load, which is much less than 230mA, we just connect it. The 230mA supply loafs at a few percent of its rating, much like a 60W lamp on home wiring.

> when i calculate it, that's what i get.

I can't make sense of your numbers. 15V/0.23A= 65 ohms. 30V/0.23A= 130 ohms. 15V/0.0053A= 2830 ohms. 30V/0.0053A= 5660 ohms. 0.0053A*353ohms is 1.9V drop. 0.23A*353ohms is 81V drop.

Show your work!!! If there is a misunderstanding, a worked example may clarify. If you made a mistake, and someone points it out, you 'feel stupid' for a minute and empowered for life.

 

[brendandwyer]

thanks guys!

it is clear, the minute i stopped thinking in overly complex terms and thought in terms of my house electrical (like you said prr) it made sense to me.

I will show my work in the future, and i will add my location.

my numbers don't make sense do they

i did a dumb thing and converted the amp to mA then tried the calculation.

i do appreciate your help.

 

[emrr]

One piece I see missing, and worth knowing.

If the supply is an unregulated type the rating can mean 'puts out +/-15V when 230mA current is drawn'. If you draw less current the supply voltage will rise some degree, and depending on the supply in question it may rise beyond the safety point for the item being powered.

But you are probably referring to a regulated supply, which will maintain voltage over a wide current swing. Sometimes you can find a number for voltage change relative to full load versus no load.

 

[brendandwyer]

you're right, that is worth knowing. I had no idea what the difference was.

The PSU i will be using is shown as a +-15vdc 230mA regulated power supply.

It has a wall transformer.

thanks!